In recent years, in the context of the concerns of power shortage and requirements for global environmental protection, power saving is required in every household, store, building or the like. For detailed example, the electrical power demand increases in summers and winters, and power consumption by every household, store and building may exceed the contract power. Thus, in order to save the power efficiently to prevent the power consumption from exceeding the contract power, a power management system by which power consumption in every household, store or building is monitored and power consumption by each electrical product (hereinafter referred to as a power load) is controlled (what is called a HEMS (Home Energy Management System)) has been proposed. In HEMS, power consumption by each power load is detected by a power sensor, and the data of the detected power consumption is collected by a master unit through wireless communication. Then, the master unit transmits a control signal to each power load corresponding to the power consumption while monitoring the power consumption, thus the power consumption by each power load can be reduced efficiently.
When the number of power loads to be controlled increases, the frequency of transmitting the power consumption data from a power sensor increases and an error is caused by congestion, thus the power consumption data may not reach the master unit. Therefore, for example, a method by which, when the communication quality declines, the power consumption data is transmitted in a transmission mode with a high resistance to an error has been devised (e.g. Patent Literature 1).